This invention relates, in general, to masks used in making semiconductors products, and more particularly, to making phase-shift masks.
At present, small features or small geometric patterns are created by using conventional optical photolithography. Typically, optical photolithography is achieved by projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Once the light areas and the dark areas are formed, they are projected onto and through a lense and subsequently onto a substrate. However, because of increased semiconductor device complexity which results in increased pattern complexity, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore, making diffraction a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase-shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase-shifting is a change in timing or a shift in wave form of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the periodic pattern of the light wave. Phase-shift masks reduce diffraction effects by combining both diffracted light and phase-shifted light so that constructive and destructive interference takes place. A summation of the constructive and destructive interference results in improved resolution and improved depth of focus.
Conventional phase-shift masks are made by a number of methods; however, most methods of making phase-shift masks require that a layer of opaque material be deposited onto an optically clear plate and then patterned. The patterned plate is then redeposited with a phase-shift material and is once again patterned, thereby making a phase-shift pattern made up of phase-shift elements. Thus, the typical method of making conventional phase-shift masks not only requires at least two patterning steps, but also requires a critical alignment of the phase-shift pattern to the opaque pattern as well. Additionally, in some fabrication methods of conventional phase-shift masks, not only is there a critical alignment step, but, in addition, a critical etch step is required. The critical etch forms the phase-shift elements in such a manner so that the phase-shift elements are slightly larger than the opaque material, thereby creating a phase-shift layer that slightly overhangs the opaque areas. This overhang is not only difficult to make and to control but also is susceptible to particle contamination, thereby ruining the conventional phase-shift mask.
Additionally, design of a conventional phase-shift mask is not an easy task because the phase-shift patterns or the phase-shift elements are not generically or symmetrically placed over the entire mask, but are placed only on certain portions of the mask. This design problem creates a pattern determination problem that is not easily solved. Typically, only a certain portion of the regularly repeating patterns receive phase-shift elements that are positioned between the opaque portions of the conventional phase-shift mask; however, by only placing phase-shift elements in the repeating patterns a large number of geometric patterns do not receive any benefit from phase shifting.
By way of example only, one type of phase-shift mask, as well as a detailed description of theory is disclosed in Marc D. Levenson et al., "Improving Resolution in Photolithography with a Phase-Shifting Mask," I.E.E.E. TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO.12, DECEMBER, 1982 which is hereby incorporated herein by reference.
Accordingly, it is desirable to make a phase-shift mask that does not have individual phase-shift elements, that does not require an additional patterning step, that does not have a critical alignment step to properly correlate the opaque pattern to the phase-shift pattern, and that does not have a critical etch step, while still achieving benefits of using a phase-shift mask.